<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="button" value="平方" onclick="f()">
<div>结果:<span></span></div>

<script>
  function f(){
    let i = document.querySelector("#i1");
    let d = document.querySelector("span");
    if (isNaN(i.value)){
      d.innerText="输入错误!"
      return;
    }
    //当两个字符串进行-*/运算时会自动转成数值,进行+运算是拼接

    d.innerText = i.value*i.value;
    //d.innerText = i.value*1+i.value*1;
    //parseFloat()将字符串转成整数或小数
    s.innerText = parseFloat(i.value)+parseFloat(i.value);
  }
</script>
</body>
</html>